Device with common mode feedback for a differential output

Abstract

A functional circuit such as an OP-amp has a differential output. A common mode signal at the differential output is adjusted by means of a common mode feedback circuit coupled between the differential output and the common mode adjustment input. The common mode feedback circuit contains IGFETs, each having a channel and a backgate, each connection of the differential output being coupled to the backgate of a respective one of the IGFETS. Thus the voltages at the outputs influence the current through the channel. The sum of the currents determines a feedback to the common mode control input.

What is claimed is: 
     
       1. A device comprising a functional circuit with connections for a differential output and a common mode adjustment input for adjusting a common mode signal at the differential output,   a common mode feedback circuit coupled between the differential output and the common mode adjustment input,   the common mode feedback circuit comprising IGFETs, each having a channel and a backgate, each connection of the differential output being coupled to the backgate of a respective one of the IGFETS so as to have an influence on a current through the channel, a sum of said influences determining a feedback to the common mode control input.   
     
     
       2. A device as claimed in claim 1, having a power supply connection for supplying power to the circuit, the IGFETs having a gate electrode connected to the power supply connection for pulling the IGFET in a conductive state, whereby the gate carries substantially a potential of the power supply connection. 
     
     
       3. A device as claimed in claim 1, comprising a circuit for forming a sum current by summing the currents through the channels of the IGFETS and for using the sum current to control an output current supplied to the common mode adjustment input. 
     
     
       4. A device as claimed in claim 3, wherein the common mode feedback circuit has at least two common mode adjustment current source outputs coupled to respective ones of the connections of the differential output, for supplying substantially identical output currents to each of the common mode adjustment outputs in dependence on the sum current through the channels of the IGFETS. 
     
     
       5. A device as claimed in claim 1, comprising further IGFETs, each connection of the differential output being coupled to the backgate of a respective one of the further IGFETs so as to have a further influence on the current through the channels of the further IGFETs, a further sum of said further influences co-determining the feedback together with the sum of said influences, the sum of the influences and the further sum of the influences having a dependence on the differential voltage at the differential output with an opposite sign, whereby said dependences on a differential voltage at the differential output at least partially compensate one another. 
     
     
       6. A device as claimed in claim 1 where the functional circuit is an differential input-output op-amp circuit. 
     
     
       7. A device as claimed in claim 3 where the functional circuit is an differential input-output op-amp circuit.